The present invention relates to water resistant, thermoplastic, starch materials which can be processed into environmentally safe products, particularly moldings and thermoplastically produced sheets as well as cast sheets.
Starch esters, particularly acetates, have been known for a long period of time. A distinction is made between highly substituted starch acetates having a degree of substitution (DS) of 2-3 and low-substituted starch acetates having a DS up to a maximum of 1. Whereas highly substituted starch acetates have not yet gained any technical importance, low-substituted starch acetates are well-established commercial products.
Known acetates having a high degree of substitution (DS 2.5-3) are cellulose and amylose triacetates. As described in the literature, the properties of films consisting of amylose triacetate are similar to those made of cellulose triacetate. These films were made of chloroform.
The process used to produce starch acetates, according to methods and common practices in the chemical industry, is also generally known and described in numerous citations (e.g., by using acetic anhydride, acetic anhydride pyridine, mixtures consisting of acetic anhydride and glacial acetic acid, ketene, vinyl acetate, and acetic acid; potato and corn starch being the predominantly used starches). Relatively long reaction times, and drastic reaction conditions, had to be accepted to obtain highly substituted derivatives.
Especially disadvantageous in connection with these processes is the strong decomposition of the starch molecules into relatively short chains which no longer have the film-forming properties typical of starch.
Little has been written about the production of high amylose starch acetate having a high degree of substitution. A paper by Mark and Mehltretter is found in U.S. Pat. No. 3,795,670 and in the corresponding publication "Facile Preparation of Starch Triacetates" in the journal Starke (1972), Issue No. 3, pages 73-100. A commercially available, high amylose corn starch from National Starch, having an amylose content of about 70%, was used as the starch. By avoiding the above conventional processes and substance components used therein, the goal of complete acetylization was obtained by the well-calculated selection of the catalyst, variation of its quantity, and with a reaction time of approximately 5 hours. The acetate which was obtained after 5 hours, having a degree of substitution of 3, could be cast together with a dichloromethane solution into flexible, transparent sheets. Although no other high amylose starches were used, it was assumed that the general process, as indicated, would be usable for high amylose starches with the same course and results, and that the resulting starch triacetates could furthermore be converted into fibers.
However, as expected, the triacetates obtained by this process, as well as sheets produced therefrom, are not fully biodegradable and/or compostable within acceptable periods of time; no advantage over cellulose acetates has been observed in respect to this point.